The invention relates to perforation devices used in wells and more particularly to a perforation device used along a production string and including charges oriented in a single direction.
Unidirectional perforation devices capable of being lowered into wells along the production string exist in many types. One type described in U.S. Pat. No. 3,965,993 (J. Lavigne, P. Chesnel and G. Bouguyon) comprises a bar forming a support on which are fixed, by their front part, encapsulated hollow charges oriented in a single direction. The performance of such a unidirectional device is excellent provided the front side of the bar is correctly applied against a side of the casing, the center-lines of the charges being directed toward this side.
During detonation, there is then a minimum spacing between the casing and the front side of the charges, this position yielding the best perforation performance. In addition, it is in this firing position that the deformation of the bar and damage to the casing are smallest and that a maximum number of charge covers remains attached to the bar after detonation.
The perforation device described in the above U.S. Pat. No. 3,965,993 is used commercially with a magnetic positioning system placed at the head of the apparatus. This system includes permanent magnets placed on the side of the supporting bar to apply the front side of the bar against the casing. It may unfortunately occur that the force of the magnets is insufficient to apply the supporting bar correctly against the casing. This application of the bar against the casing can be incorrect if the magnets have lost their force or in the case of a large number of charges placed on a long support. The addition of another magnetic positioning system at the bottom of the support is not usually practical because the debris of the charge cases agglomerates, after firing, on this magnetic element, producing a risk of jamming when the bar and the remaining charge covers are brought back up to the surface.
Another positioning system for a perforation device of the same type is described in U.S. Pat. No. 3,858,651. Springs in the form of a safety pin drive the supporting bar of a perforator against a side of the casing while bearing on the other side. Such a system involves jamming risks in the casing or the production string. In addition, after firing, debris may accumulate on the lower spring which closes off part of the casing. This possible accumulation of debris increases the risk of jamming.